Hurricane Rosa Expected to Impact Arizona

Forecast Discussion:

Hurricane Rosa is a major hurricane currently located more than 1,000 km west of the central Mexican coastline. The storm is moving slowly westward at the moment, but will begin to recurve northward tomorrow, then accelerate toward the north-northeast by Sunday, steered by high pressure to the east over Mexico. Meanwhile, an unusually deep upper-air trough for this time of year will develop along the West Coast and direct south-southwesterly flow and bring the hurricane towards the Southwestern U.S. The hurricane will likely be downgraded to a tropical storm before it makes landfall somewhere along the northern half of the Baja of California on around Monday, but some of the forecast models are indicating that the remnants of the storm will move across Arizona by next Tuesday. Because the storm will link up with the divergent jet stream winds associated with the approaching upper-air trough, it may allow the storm to maintain some strength and produce some windy conditions next Tuesday, especially in southern Arizona.

There is considerable uncertainty in the precise path and timing of Rosa, but what is more certain is that a very moist airmass will spread across nearly all of Arizona on Monday and Tuesday, with considerable lifting in a divergent part of the jet stream and with a tropical airmass moving up over the terrain of Arizona. Thus, the National Weather Service is now indicating perhaps 3-4” of rain on Monday to Wednesday morning for the Quad City Area (see attached briefing). Temperatures will also be about 10 degrees cooler next week than they have been this week.

Expect the moisture content of the air to start increasing dramatically on Sunday afternoon and continuing to Monday morning, with an increasing chance of showers and thunderstorms, some with very heavy rain and isolated storms with up to 0.5”-1” hail possible. During the day on Monday and through Tuesday as the core of the storm possibly moves through Arizona, the atmosphere will become less unstable but with continued lifting and very saturated air, widespread moderate to heavy rain may develop and persist over many areas. The biggest impacts will be the potential for flash flooding.

Scattered rain showers or thunderstorms will continue to develop and propagate north-northeastward across the area on Wednesday and Thursday, then begin to taper off. However, there could be more rain on tap for ERAU’s OctoberWest weekend. Stay Tuned.

C. James

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